This invention relates to computer emulation of sequential data storage, and more particularly, to a method for transferring data between a host computer and a random access storage device via sequential access command protocol.
Two types of formats are typically used with computers to store data. One format is referred to as sequential access and the other as random access.
In sequential access format (see FIG. 1), data is stored serially on a sequential media such as a magnetic tape in sets of data blocks, each block typically comprising 512 bytes. Between each set of data blocks is an overhead media area which contains such information as a file mark. Associated with each set of data blocks is identification information and a cyclic redundancy check (CRC). The file mark indicates an adjacent set of data block's sequential relationship to other data blocks. Other areas of the media include information indicating a beginning of media block, an end of the media block, and an early warning area. The beginning of media block is the starting location where data can be stored, while the end of media block is the last location where data can be stored. An early warning area indicates that an end of media block is approaching when the media is scanned.
In a typical random access format, data is stored in defined areas on the media. Another separate area is provided on the media containing a directory (file allocation table or FAT) indicating where the stored data is located. Because the sequential access media requires that overhead and other information be stored in a predetermined configuration and because this predetermined configuration is not present with the random access media, the two media's are incompatible.
Sequential data storage devices, such as magnetic tape streamers, are widely used with computers to back-up information stored on hard disk drives. The tape streamers are fed information from the host computer in compliance with ANSI SCSI specifications for sequential data. In the event of a failure of the hard disk drive, the backed up information on the tape is transferred back to a repaired hard drive.
With the advent of new high density storage medias such as magneto-optical disks (which is a random access media), the use of serial data storage devices may diminish. However, there is a large contingent of users that have tape streamers controlled by tape based management software which, unfortunately, is not compatible with random access devices. Thus, it would be advantageous to the large contingency of tape-based users if they could access data on a random access storage device with tape based management software.
Sequential data storage devices hold a large amount of data on the media with the storage limitations being the length of the tape. A drawback to a sequential data storage device is that typically between 20 and 200 seconds are required to access data as the device must forward the media to the location where the information to be retrieved is stored. Thus, the larger the media the larger the potential access time. Another drawback to sequential data devices is that they must be rewound to the beginning of tape before a new data storage is initiated, thereby increasing overall access and storage time.